Wind turbines used to generate wind power include horizontal axis wind turbines in which the axis of rotation is positioned parallel to the earth's surface and vertical axis wind turbines in which the axis of rotation is positioned perpendicular to the earth's surface. In addition, vertical axis wind turbines are classified roughly as either a lift type, in which the wind turbine is caused to rotate by a lift force acting on the blades, or as a drag type, in which the wind turbine is caused to rotate by a drag acting on the blades. As a conventional drag type of wind turbine, the type disclosed in Unexamined Japanese Patent Application Kokai Publication No. 2006-46306 has been known from before.
With a conventional drag type wind turbine, because the rotational velocity is determined so as to be proportional to wind speed, especially during very high winds such as typhoon-class winds, there are concerns that the rotational velocity could become excessively great, causing equipment damage and deformation. In addition, when the number of rotations of the wind turbine during all wind speeds is controlled in order to avoid this kind of situation, the problem arises that efficiency is lost in a medium wind speed range and startability is lost in a low wind speed range.
In consideration of the foregoing, it is an object of the present invention to provide a blade, a wind turbine and a wind power generating device that can achieve good startability during low wind speeds (i.e. a range of wind speeds classified as low) and high energy conversion efficiency during medium winds (i.e. a range of wind speeds classified as medium), while also preventing velocity from becoming excessively great during high winds (i.e. a range of wind speeds classified as high).